1. Field of the Invention
This invention relates to rubber compositions for tread in high-speed running tire, and more particularly to a tread rubber composition for high-speed running tires having improved heat resistance and wear resistance.
2. Related Art Statement
In high-speed running tires such as general-purpose passenger car tire running at a speed of not less than 100 km/h on expressways or the like, sport car tire, motor sport tire, racing tire and so on, the tread rubber is subjected to dynamic compression deformation from road surface at a high frequency during the high-speed running, so that the heat build-up is large, and particularly it reaches to a temperature of not lower than 200.degree. C. at a tread portion showing an extremely high ground contact pressure and a strain concentrated portion due to ununiform deformation in a block pattern formed in the tread to raise a risk of blow-out, and consequently it is required to have a sufficient heat resistance.
Especially, the tread rubber of the high-speed running tire is required to ensure a stable steering stability that a will of a driver is surely transmitted to a road surface. For this end, it is necessary that the resistance to slipping friction between road surface and tread rubber is made large to sufficiently overcome inertia force of momentum in car at the braking, centrifugal force at the cornering and driving force of tire at the starting. In order to make the resistance to slipping friction large, it is required to make the hardness of the tread rubber small so as to enhance the ground contacting area between road surface and tread rubber. In this case, the deformation strain of rubber due to the ground contact pressure from road surface becomes rather large, and consequently the heat build-up increases to enhance the risk of blow-out. From this point, the tread rubber for high-speed running tire is demanded to have a severer heat resistance.
In general, a method of adding an aromatic oil to rubber is carried out as a means for reducing the hardness of the tread rubber to improve the gripping property of the tire. However, this method degrades the wear resistance and fracture strength to create many breakages of the tread rubber (cut, chipping), so that it is unsuitable for the high-speed running tire.
As a conventional means for improving the heat resistance of rubber, there is a method wherein the number of double bonds in the polymer itself is reduced as far as possible. In this case, however, the stretchability of rubber itself is lost to produce a rigid rubber, so that this method is also unsuitable for the high-speed running tire.
Further, there are a method wherein polysulfide crosslinked networks are decreased as far as possible and monosulfide crosslinked networks having a strong bonding force are increased by increasing the amount of vulcanization accelerator and reducing the amount of sulfur to improve the heat resistance of rubber composition, and a method of directly forming strong C--C bond between polymers as in peroxide vulcanization to largely improve the heat resistance. The former method certainly improves the heat resistance, but is still insufficient in the heat resistance required in the high-speed running tire. The latter method largely improves the heat resistance, but loses the stretchability of rubber to considerably degrade the mechanical fatigue properties, so that it is not suitable for the high-speed running tire.